1. Field of the Invention
The present invention generally relates to photosensitive polyimides and, more particularly, to a polyamide acid precursor with photosensitive groups bonded to the carboxylic acid moieties.
2. Description of the Prior Art
Polyimides are condensation polymers derived from bifunctional carboxylic acid anhydrides and primary diamines. Aromatic polyimides exhibit outstanding mechanical properties for application in microelectronics manufacture and have excellent thermal and oxidative stability. It is well known that the mechanical properties, thermal stability, adhesion, swelling, morphology, dielectric constant, coefficient of thermal expansion, and residual stress are determined to a large extent by the choice of the diamine and dianhydride components used as starting materials. Polyimide materials which are currently commonly used in microelectronics manufacture are synthesized from the monomers noted in Table 1.
TABLE 1 ______________________________________ Dianhydrides Diamines ______________________________________ 1) 3,3'4,4'-biphenyltetra- 1) M- and P-phenylene- carboxylic acid diamine (PMDA) dianhydride (BPDA) 2) pyromellitic 2) 4,4'-diaminodiphenyl dianhydride ether (PMDA) (ODA) 3) benzophenone 3) 4,4'-diaminodiphenyl dianhydride methane (MDA) (BTDA) ______________________________________
During manufacture of thin film structures, whether for chip or single or multi chip module packaging, the removal of solvents from the solvent based coatings as well as subjecting the films to numerous heating and cooling regimes for curing purposes generates residual stresses within the films. The residual stresses produced by the difference in the thermal expansion coefficients of the materials and the moduli of the various material layers can lead to film delamination, cracking of the metal or polyimide, and bowing of the substrates resulting in poor photolithographic image transfer. In view of these problems, low stress materials are highly desirable for thin film fabrication.
Pattern generation in polyimides is normally achieved using photolithographic processes where patterns are transferred to the polyimide using lift-off techniques. The disadvantage of these processes is that multiple processing steps are required. In order to reduce process steps and cost in high density chip wiring and integrated device packaging, photosensitive polyimides have gained much attention in the electronics industry. Photosensitive polyimides have the potential of allowing the manufacture of multi-layer packaging of high performance integrated circuit devices in small and dense scale with less processing steps.
However, many of the currently available photosensitive polyimides have poor mechanical properties, such as very low elongation break, low modulus, low tensile strength, and brittleness, high thermal expansion, relatively high dielectric constant, high swelling in solvent, poor thermal stability and low glass transition temperature. Bateman, U.S. Pat. No. 4,830,953, Pfeifer et al., U.S. Pat. No. 4,698,295, Mueller et al., U.S. Pat. No. 4,803,147, and Pfeifer, U.S. Pat. No. 4,629,777, all disclose photosensitive polyimides. However, there are many limiting factors associated with these products. For example, Bateman, Pfeiffer et al. and Mueller are only applicable to thin film use. In addition, all four of these products have poor photoresolution. Other problems associated with these products include poor thermal stability, high dielectric constant, poor adhesion, and poor mechanical properties especially after thermal cycling associated with multilayer thin film fabrication. Rohde et al., U.S. Pat. No. 4,656,116, discloses photosensitive polyimides which include aromatic tetracarboxylic acid derivatives such as BTDA and PMDA, and alkyl substituted aromatic diamines where the alkyl groups can be radiation crosslinked with organic chromophoric polyazides. However, the Rohde et al. polyimides lack many of the critical properties needed for circuit manufacture and have low photosensitivity and yield poor pattern resolution in film thickness required for thin film packaging. The photo-patterning results discussed in Rohde et al. were obtained at very thin film thickness (ca. 1-2 .mu.m).